Polarisation is the direction in which the electric field of an EM wave oscillates. The antenna determines the polarisation of the transmitted signal — and an antenna receives waves with the "wrong" polarisation poorly or not at all.
- Horizontal polarisation: the E field oscillates horizontally.
- Vertical polarisation: the E field oscillates vertically.
- Circular polarisation: the E field rotates like a corkscrew around the direction of propagation (left- or right-handed).
Why this matters for radar:
- Suppression of rain clutter: raindrops are almost spherical and reflect circularly polarised waves with the opposite sense of rotation; the radar's own antenna hardly receives them. Complex targets such as aircraft do disturb the sense of rotation and remain visible. Many radars therefore switch to circular polarisation in precipitation.
- Target characteristics: how a target reflects different polarisations says something about its shape and orientation (polarimetry).
- EW application: a jammer is most effective with the same polarisation as the victim radar; cross-polarisation can waste part of the jamming power — or be used deliberately to deceive monopulse trackers.
In depth: polarisation loss from mismatch
If the polarisation of the receiving antenna does not match that of the incoming wave, power is lost. For two linear polarisations with an angular difference θ, the Polarization Loss Factor (PLF) applies:
| Mismatch | Received power | Loss |
|---|---|---|
| Matched (0° — V/V or H/H) | 100% | 0 dB |
| 30° difference | ≈ 75% | ≈ 1.25 dB |
| 45° difference | 50% | 3 dB |
| 60° difference | 25% | ≈ 6 dB |
| Linear ↔ circular | 50% | 3 dB |
| Orthogonal linear (90° — V/H) | ≈ 0% | theoretically ∞ (in practice 20–40 dB) |
| Opposite circular (RHCP/LHCP) | ≈ 0% | theoretically ∞ |
The two extremes drive many design choices: 45° always costs exactly half (3 dB), and the transition linear↔circular also costs 3 dB — the price you pay for the circular "rain mode" above, because on ordinary (linearly reflecting) targets you then lose 50%.
A jammer with the wrong polarisation (e.g. 90° cross-pol against a matched radar) loses almost all its power — unless that is exactly the intention: cross-polarization jamming (Cross-polarization) deliberately exploits the distorted cross-polar "Condon lobes" of the antenna to corrupt monopulse angle measurement. Polarisation is therefore both a source of loss and a weapon.